On the behaviour of Ru(I) and Ru(II) carbonyl acetates in the presence of H2 and/or acetic acid and their role in the catalytic hydrogenation of acetic acid

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On the behaviour of Ru(I) and Ru(II) carbonyl acetates in

the presence of H

2

and/or acetic acid and their role in the

catalytic hydrogenation of acetic acid

Antonella Salvini, Piero Frediani

*

_{, Carlo Giannelli, Luca Rosi}

Department of Organic Chemistry, University of Florence, Via della Lastruccia, 13-50019 Sesto Fiorentino, Italy Received 9 September 2003; accepted 21 September 2004

Abstract

The reactivity of phosphine substituted ruthenium carbonyl carboxylates Ru(CO)2(MeCOO)2(PBu3)2, Ru2(CO)4

(l-MeCOO)2(PBu3)2, Ru4(CO)8(l-MeCOO)4(PBu3)2with H2and/or acetic acid was investigated by IR and NMR spectroscopy to

clar-ify their role in the catalytic hydrogenation of acetic acid. Evidences were collected to suggest hydride ruthenium complexes as the catalytically active species. Equilibria among ruthenium hydrides and carboxylato complexes take place in the presence of hydrogen and acetic acid, that is in the conditions of the catalytic reaction. Nevertheless the presence of acetic acid reduces the rate of the formation of hydrides. Working at a very high temperature (180°C) polynuclear phosphido hydrides such as [Ru6

(l-H)6(CO)10(l-PHBu)(l-PBu2)2(PBu3)2(l6-P)] were formed. These phosphido clusters are suggested as the resting state of the catalytic

system.

Furthermore the bi- or tetranuclear Ru(I) carboxylato complexes react with acetic acid giving a mononuclear ruthenium complex Ru(CO)2(MeCOO)(l-MeCOO)(PBu3), containing a monodentate and a chelato acetato ligands. This complex was spectroscopically

characterised. Its identity and structure were conﬁrmed by its reactivity with stoichiometric amount of PPh3to give Ru(CO)2

(Me-COO)2(PBu3)(PPh3), a new mononuclear ruthenium carbonyl carboxylate containing two diﬀerent phosphines, that was fully

characterised.

Keywords: Ruthenium; Carbonyl carboxylates; Acetic acid; Hydrogenation

1. Introduction

The catalytic hydrogenation of carboxylic acids in homogeneous phase has been investigated in our labo-ratory for several years[1]. This reaction was achieved using ruthenium complexes such as Ru4H4(CO)8

(P-Bu3)4 (1) and Ru4H4(CO)8[()-DIOP]2 [2] as catalytic

precursors.

Ruthenium carbonyl carboxylates Ru(CO)2

(Me-COO)2(PBu3)2 (2), Ru2(CO)4(l-MeCOO)2(PBu3)2 (3),

Ru4(CO)8(l-MeCOO)4(PBu3)2(4), were detected in the

crude of the hydrogenation of acetic acid in the presence of (1) as catalytic precursor [2a,3]. These carboxylato complexes (2)–(4) are catalytically active in the hydro-genation of acetic acid[1b].

The behaviour of (1)–(4) with hydrogen has been studied [1d,1e,1h] in order to understand their role in the catalytic hydrogenations. The ruthenium carboxy-lato complexes (2) and (3) react with hydrogen at low temperature (50 and 100 °C, respectively) to give the hydrido complexes RuH2(CO)2(PBu3)2 (5), Ru4H4

-(CO)9(PBu3)3 (6) and (1) [1h]. At higher temperature

(over 140 °C), the phosphido ruthenium clusters

* _{Corresponding author. Tel.: +390554573522; fax: +390554573531.}

E-mail address:piero.frediani@uniﬁ.it(P. Frediani).

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[Ru6(l-H)6(CO)10(l-PHBu)(l-PBu2)2(PBu3)2(l6-P)] (7),

[Ru6(l-H)6(l-CO)(CO)12(l-PBu2)(PBu3)2(l6-P)] (8), [Ru7

-(l-H)8(CO)12(l3-PBu)(l-PBu2)(PBu3)2(l6-P)] (9), [Ru3

-(l-H)2(CO)7(l3-PBu)(PBu3)2] (10) are formed [1d,1e]

(Scheme 1).

To get more information on the role played by the complexes (2–4) in the catalytic hydrogenation of acetic acid we have now investigated the behaviour of these complexes in the presence of acetic acid or acetic acid and H2. The reactions were monitored by ‘‘in situ’’

HP-IR spectroscopy, using a cell directly connected to the reaction vessel. Samples of the solutions were also analysed by glc and glc-ms to identify the organic prod-ucts formed. The new ruthenium complexes were char-acterised by IR and NMR spectroscopy.

Some reactions were also monitored by 31P NMR spectroscopy.

At the end of this investigation, the hydrogenating activity of [Ru6(l-H)6(CO)10(l-PHBu)(l-PBu2)2(PBu3)2

-(l6-P)] (7) has been tested in order to evaluate the role

played by the phosphido clusters in the catalytic hydro-genation of acetic acid.

2. Results and discussion

2.1. Reactivity of ruthenium carbonyl carboxylates with acetic acid

2.1.1. Ru2(CO)4(l-MeCOO)2(PBu3)2(3)

2.1.1.1. IR study. Ru2(CO)4(l-MeCOO)2(PBu3)2(3), in

n-heptane as solvent, reacted with acetic acid (Ru/ CH3COOH = 1:30) at 40°C giving a new complex

Ru-(CO)2(MeCOO)(l-MeCOO)(PBu3) (11) as evidenced

by new bands at 2060(ﬀ), 1990(ﬀ) and 1575(m) cm1. After 48 h traces of (3) were still present. A quantitative conversion of (3) into (11) was reached after 70 h.

No other transformations were observed increasing the temperature up to 120 °C. At this temperature part of the acetic acid was present in vapour phase decreasing its concentration in solution. As a consequence the reac-tion was reversed and (3) returned to be the main complex present in the solution (Scheme 2). A further increase of temperature (150°C) reduced further the concentra-tion of acetic acid in soluconcentra-tion and the amount of (11).

The process is reversible because when the vessel was cooled to room temperature and the concentration of acetic acid restored to its initial value, the amount of (11) increases: after 48 h (11) was the sole ruthenium complex present in solution.

In the residue obtained removing the solvent and the acetic acid at reduced pressure and low temperature (10 °C), the complexes (11) and (2) were evidenced by IR spectroscopy using n-pentane as solvent. However, the IR spectrum of the same residue in a n-heptane/ace-tic acid solution showed the presence of (11) as the sole complex in solution because the absorptions of (2) in acetic acid are shifted and overwhelmed by those of (11). Complex (11) showed a low stability in a hydrocar-bon solution in the absence of acetic acid.

2.1.1.2. NMR study. The reactivity of (3) with acetic acid was also carried out in an NMR sample tube using C6D6as solvent. After an hour at room temperature a

singlet at 43.5 ppm (11.7%) attributable to (11) and a singlet at 17.2 ppm (1.2%) due to (2) were present in the 31P NMR spectrum. Complex (2) was not easily identiﬁed in the IR spectrum performed in a C6D6/acetic

acid solution when (11) was the main ruthenium com-plex because the absorptions of (2) in the presence of acetic acid are overwhelmed by those of (11). However, if the acetic acid was removed and the residue dissolved in n-pentane, the presence of (11) and (2) might be easily evidenced (see above).

A total conversion of (3) was reached after 56 h at 40°C [70.4% of (11) and 29.6% of (2)]. No other signals were present in the31P NMR spectrum.

The 31P-, 1H-, and 13C NMR spectra (C6D6 as

sol-vent) of the residue after elimination of the acetic acid conﬁrmed the presence of (11) and (2). The NMR reso-nances were shifted with respect to those collected from the solution containing acetic acid. The presence of (2) was conﬁrmed by its characteristic resonances. The com-plex (11) showed resonances due to PBu3(a broad

sin-glet at 44.4 ppm in the 31P NMR spectrum) and CO ligands (195.9 (m) ppm in the 13C NMR spectrum), two inequivalent MeCOO groups (174.5 and 183.0 ppm in the13C NMR spectrum attributable to a mono-and a bidentate acetato ligmono-and, respectively). These attributions are in agreement with the resonance of the

Ru2(CO)4(MeCOO)2(PBu3)2

Ru(CO)2(MeCOO)2(PBu3)2

Ru4H4(CO)8(PBu3)4

RuH2(CO)2(PBu3)2

Ru4H4(CO)9(PBu3)3 (2) (3) (1) (5) (6) 50˚C 100˚C H2 H2 T>140˚C (7) (8) (9) (10) Ru₆H₆(CO)₁₀(PHBu)(PBu₂)₂(PBu₃)₂(P)

Ru7H8(CO)12(PBu)(PBu2)(PBu3)3(P)

Ru6H6(CO)13(PBu2)(PBu3)2(P)

Ru3H2(CO)7(PBu)(PBu3)2

H2

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monodentate acetato group in (2) (175.9 ppm) and the bidentate acetato ligand in Ru(l-MeCOO)2[(S)-BINAP]

(188.1 ppm)[4], respectively. The broad signal in the31P NMR spectrum may be due to a rapid intramolecular rearrangement between the acetato ligands. Several complexes containing both mono- and bidentate carb-oxylato ligands are reported, such as Ru(CO)-(RCOO)(l-O,O0_-RCOO)(PPh

3)2 (R = Me, p-C6H4Cl,

p-C6H4NO2, CF3, C2F5, C6F5)[5]. A rapid

intramolecu-lar exchange between mono- and bidentate carboxylato ligands was observed by NMR studies on these com-plexes (R = Me, CF3, C2F5, C6F5) [6] at variable

temperature.

The ﬂuxional structure of the complex (11) was con-ﬁrmed by the1H NMR spectrum. The broad resonance at 2.02 ppm, attributed to the methyl group of the ace-tato ligands, suggests a fast intramolecular rearrange-ment of the two acetato ligands. An analogous broad signal in the1H NMR spectrum is reported by Spencer and Wilkinson [5a] for the non-rigid Ru(MeCOO)(l-MeCOO)(CO)(PPh3)2complex.

The low stability of (11) in the absence of acetic acid was conﬁrmed by several signals present in the 31P NMR spectrum recorded after 24 h.

The formation of (11) is rationalised in Scheme 2. Acetic acid is added to (3) breaking the acetato bridge

and giving an intermediate complex [Ru2(CO)4

(Me-COO)2(MeCOOH)2] containing two monodentate

carb-oxylato ligands. An analogous process involving the formation of a Ru2(MeCOO)2(CO)6(PBu3)2containing

monodentate acetato ligands has been suggested in the reaction of (3) with CO [1f] as a preliminary step of the formation of Ru(CO)4(PBu3).

The complexes that we are now proposing are clo-sely related to those previously isolated by Rotem et al. [7], containing acetic acid units directly bound to the metal.

This intermediate Ru(I) complex gives the mononu-clear Ru(II) complex (11) and H2(Scheme 2). A similar

reactivity of the coordinated acetic acid was reported to explain the synthesis of (2) from [Ru2(CO)4

(l-Me-COO)2]n (12) [8]. Ruthenium (I) was oxidised by the

coordinated acetic acid and molecular hydrogen was formed. Fachinetti et al. [9]also suggest an analogous oxidation of Ru(0) to Ru(I) in the presence of CF3COOH.

The partial transformation of (11) into (2) was ob-served by IR and NMR spectroscopy of the solution while the contemporary formation of (12), the ruthe-nium species without phosphinic ligands, was identiﬁed through IR spectroscopy (KBr pellets) on the yellow res-idue present in the NMR sample tube.

Ru Ru PBu3 Bu3P O O O _O C Me Me C CO CO CO CO (3) 120˚C PBu3, MeCOO Ru PBu3 CO CO PBu3 MeCOO MeCOO (2) 120˚C PBu3 Ru2(CO)4(MeCOO)2 n (12) MeCOO Ru Ru PBu3 Bu3P O C Me Me CO CO CO CO OCOMe MeCOO OH OH O C MeCOO PBu3 CO O Ru O CO C Me (11) H2 MeCOOH 40˚C 120˚C Scheme 2.

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When the reaction was prolonged for a long time at 120°C the Ru(II) complexes, (11) and (2), were reduced to the Ru(I) complex (3) (Scheme 2). An acetato radical may be involved. It reacted with n-heptane giving acetic acid and heptenes as showed by glc. No Ru(0) species were detected. The rearrangement of (2) into (3) at 120 °C was previously reported [1d] and attributed to a thermal transformation. On the contrary a dispropor-tionation of a polymetallic Ru(I) complex into Ru(0) and Ru(II) species was reported by Fachinetti et al.

[9,10].

2.1.1.3. Synthesis and structure of Ru(CO)2

(Me-COO)2(PBu3)(PPh3) (13). Complex (11) reacted at

room temperature with a stoichiometric amount of PPh3 giving Ru(CO)2(MeCOO)2(PBu3)(PPh3) (13)

(Scheme 3), a ruthenium carbonyl carboxylate contain-ing two diﬀerent phosphines. This new complex was iso-lated and spectroscopically characterised.

The data collected are in agreement with an octahe-dral structure containing a tributylphosphine trans to a triphenylphosphine, two cis carbonyl groups and two cis acetato ligands (Fig. 1(a)).

The other possible structures (b)–(f) (Fig. 1) have been ruled out according to the following considera-tions: the31P NMR spectrum shows an AB spin system

with resonances at 23.9 (d, 1P, PBu3, JPP= 311.6 Hz)

and 30.5 (d, 1P, PPh3, JPP= 311.6 Hz) ppm. The JPP

va-lue is in agreement with a trans coupling between the two phosphine[11]. The pattern of this NMR spectrum has been well simulated using the data above reported. The 1H- and 13C NMR spectra are in agreement with two equivalent acetato ligands ruling out the structures (13c), (13d) and (13e). The pseudo triplet at 198.8 ppm in the 13C NMR spectrum may be ascribed to the car-bon atoms of two equivalent carcar-bonyl groups coupled with two cis phosphines. As a consequence the struc-tures (13c), (13d) and (13f) showing two non-equivalent carbonyl groups must be excluded. The structures (13b) and (13e) containing the carbonyl groups in a trans posi-tion must be also discarded because two bands of equal intensity are present in the carbonyl stretching region of the IR spectrum.

The MS spectrum shows a pattern of peaks centred at m/z 740 in agreement with the Ru(CO)2(MeCOO)2

-(PBu3)(PPh3) formulation of (13).

The elemental analysis conﬁrm the reported formulation.

The synthesis of (13) from (11) and PPh3 conﬁrms

also the structure attributed to (11). 2.1.2. Ru4(CO)8(l-MeCOO)4(PBu3)2(4)

2.1.2.1. IR study. The complex (4), in n-heptane as solvent, reacted at room temperature with an excess of acetic acid (Ru/CH3COOH = 1:23) giving

(conver-sion 60% after 24 h) a mixture of ruthenium com-plexes. The new bands at 2060 and 1990 cm1 were attributed to (11) while a band at 1999(w) cm1 sug-gests the presence of a trace of another unidentiﬁed product. A broadband was also present in the carboxy-lato region at 1580 cm1suggesting the presence of an acetato complex. A reasonable mechanistic pathway for the conversion of (4) into (11) is proposed in

Scheme 4 taking into account the reactivity of (4) with CO [1f]. A preliminary coordination of acetic acid is followed by a cleavage of the oxygen bridges between two Ru atoms giving the complex [Ru2(CO)4

(l-Me-COO)2(MeCOOH)(PBu3)] (Scheme 4). In a subsequent

step this complex reacts with acetic acid giving the complexes (11) and (12). This last complex was identi-ﬁed as a yellow residue after cooling the vessel.

At higher temperature (120°C after 24 h) (11) was the sole complex present in the solution as shown by the IR spectrum.

2.1.2.2. NMR study. The reaction of (4) with acetic acid was also carried out in a NMR sample tube (C6D6 as solvent) and monitored by 31P NMR

spectroscopy.

At room temperature (11) was formed (conversion 24.5% after 1 h) together with trace of two phosphinic compounds (singlet at 13.4 and 38.8 ppm in the 31P

Ru PBu3 CO CO MeCOO (11) Ru PBu3 CO CO MeCOO MeCOO (13) PPh3 O O C Me + PPh3 Scheme 3. Ru PBu₃ PPh3 MeCOO MeCOO CO CO Ru PBu3 PPh₃ MeCOO CO OC OCOMe (13a) (13b) Ru PBu₃ PPh₃ MeCOO MeCOO CO CO Ru PBu3 PPh₃ MeCOO CO (13c) (13d) Ru PBu3 PPh3 MeCOO CO Ru PBu₃ PPh3 MeCOO CO OC OCOMe (13e) (13f) OC MeCOO MeCOO _CO Fig. 1.

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NMR). After 8 days (11) was the main complex (75.6%) present in the solution and (4) was still present (12.3%). A yellow insoluble product was also formed and identi-ﬁed by IR spectroscopy as (12).

The conversion of (4) was complete after 15 days at room temperature and (11) was the main product in the solution (87.2%) together with an unidentiﬁed prod-uct having a singlet at 38.8 ppm (12.8%) in the 31P NMR.

The IR and NMR spectra of the residue obtained after distillation of the solvent and acetic acid at low temperature and reduced pressure showed (11) as the main ruthenium species. This solution was employed to perform the spectroscopic characterisation of (11) (see Section 4).

2.1.3. Ru(CO)2(MeCOO)2(PBu3)2(2)

Complex (2) did not react with acetic acid at room temperature in a n-heptane solution (Ru/ CH3COOH = 1:30). Only a shift of the carbonyl

stretchings of (2) to higher frequencies was due to the presence of acetic acid. This behaviour is reversible: if the liquids were removed and the residue dissolved in n-heptane the starting spectrum of (2) was restored. In the same way the resonances of (2) were shifted when the1H-, 31P- and13C NMR spectra were recorded in a MeCOOH/C6D6 solution (Table 1). The multiplicity

and the coupling constants remain unchanged in all cases.

A temperature of 140°C was required to observe the conversion of (2) into traces of (3); the amount of (3) in-creased after heating at 150°C, as expected, according to the thermal stability of (2)[1d].

2.2. Reactivity of ruthenium carbonyl carboxylates with hydrogen and acetic acid

The reaction of ruthenium complexes (2–4) with hydrogen and acetic acid, that is in the conditions em-ployed for the hydrogenation of acetic acid, was tested by IR spectroscopy. The organic products were identi-ﬁed and quantiidenti-ﬁed by glc and glc-ms analyses.

2.2.1. Ru2(CO)4(l-MeCOO)2(PBu3)2(3)

The complex (3), in a n-heptane solution, did not re-act with acetic acid (Ru/CH3COOH = 1:30) and

hydro-gen (100 atm) at room temperature after a long time (15 days).

The IR spectrum of (3) was unchanged even if the solution was heated up to 140°C; however in these con-ditions ethyl acetate was formed through the hydrogen-ation of acetic acid.

Only at 180°C after 24 h (3) was transformed into the hydrido clusters (7), (9) and (10). The same ruthenium complexes were formed at lower temperature from (3) and hydrogen (50 atm)[1e]. The presence of acetic acid reduces the rate of the transformation of (3) into phos-phido ruthenium clusters.

This behaviour supports the hypothesis that interme-diate hydrido ruthenium complexes must be involved in these transformations [1h]. The presence of acetic acid decreases the rate of the hydrogenolysis of the acetato group in (3) because this reaction involves the formation of ruthenium hydrides and acetic acid. Furthermore, the presence of hydrogen hinders the formation of (11) by reaction of (3) and acetic acid, in agreement with the mechanism reported in Scheme 2.

Ru Ru Bu3P O O O O C Me Me C CO CO COCO (4) Ru2(CO)4(MeCOO)2 _n (12) Ru Ru Bu3P O C Me Me CO CO COCO OCOMe MeCOO OH OH O C MeCOO PBu3 CO O Ru O CO C Me (11) MeCOOH Ru Ru Bu3P O O O O C Me Me C CO CO COCO Ru Me Me O Ru O C O O C CO CO CO CO PBu3 r.t. MeCOOH 2 O C Me OH O C Me OH 2 2 H2+ Scheme 4.

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2.3. Ru4(CO)8(l-MeCOO)4(PBu3)2(4)

The complex (4), in n-heptane as solvent, reacted in the presence of acetic acid (Ru/CH3COOH = 1:23) and

hydrogen (100 atm at room temperature) at 60 °C giv-ing (3) (traces after 72 h) as reported inScheme 5. The complex (12), formed as a concomitant product, was recovered as a yellow solid and identiﬁed by IR spectroscopy.

The same reaction takes place in a large extent at 60°C in the presence of only hydrogen [1f]or through a thermal rearrangement of (4) at 150°C[8,12].

These data support the hypothesis that the phosphine redistribution also takes place through hydride interme-diates, formed in an undetectable amount. The forma-tion of ruthenium hydrides is reduced by the presence of acetic acid and consequently the phosphine redistri-bution is very low. It was necessary to heat (4) at 140 °C for 20 h to obtain a (4)/(3) molar ratio of 3:2.

The complex (4), heated at 160 °C for 25 h, was al-most completely transformed into (3) and (12). Ethyl acetate was also present in the solution collected at this temperature and analysed by glc.

2.4. Ru(CO)2(MeCOO)2(PBu3)2(2)

The complex (2), in a n-heptane solution, did not re-act with acetic acid (Ru/CH3COOH = 1:30) and

hydro-gen (100 atm) at room temperature. Only a shift of the bands in the IR spectrum was shown as observed under nitrogen.

Heating this solution for 24 h at 80°C the complexes (5) [1d]and RuH(CO)2(MeCOO)(PBu3)2(14)[13]

[con-version of (2) 56%; (5)/(14) = 1/1 molar ratio] were formed. Traces of (3) were also formed after 48 h at this temperature.

The conversion of (2) increased when the solution was heated at higher temperature; the dihydride (5) was the main ruthenium complex (49%) after 24 h at 100°C [conversion 79%; (5)/(14) = 1.6 molar ratio] and (5) became 65% at 140 °C after 24 h [conversion 94%; (5)/(14) = 2.2 molar ratio].

At 180°C, (5) (78%) and (14) (22%) were present to-gether with traces of (2) and (3): at this temperature ethyl acetate was also formed by reduction of acetic acid.

The rate of the transformation of (2) into (5) was re-duced by the presence of acetic acid according to the equilibria reported inScheme 6.

The formation of (14) as intermediate of the reaction of (2) with hydrogen has been previously reported work-ing at low temperature (50 °C) in the presence of Na2CO3[1h].

The presence of an excess of acetic acid reduce the rate of the formation of (5) and, as a consequence, in-creases the amount of (14).

Ru4(CO)8( MeCOO)µ− 4(PBu3)2 Ru2(CO)4( MeCOO)µ− 2(PBu3)2 + 1/n[Ru2(CO)4( MeCOO)µ− 2]

(4) (3) (12)

n

Scheme 5. Table 1

Spectroscopic data of Ru(CO)2(MeCOO)2(PBu3)2a: inﬂuence of acetic acid

Solvent IR m (cm1) Solvent 31_{P NMR (ppm)} 1_{H NMR (ppm)} 13_{C NMR (ppm)} n-Heptane 2041(vs) C6D6 17.4 0.86 (t, 18H, CH3CH2, JHH= 7.3 Hz), 1.28 (q, 12H, CH3CH2, JHH= 7.3 Hz), 1.50 (m, 12H, CH2CH2P), 1.86 (m, 12H, CH2P), 2.25 (s, 6H, CH3COO) 13.8 (s, CH3CH2), 23.7 (s, CH3COO), 23.9 (t, CH2P, JCP= 12.7 Hz), 24.8 (t, CH3CH2, JCP= 6.3 Hz), 25.4 (s, CH2CH2P), 175.9 (s, CH3COO), 199.0 (t, CO, JCP= 10.9 Hz) 1971(vs) 1628(m) n-Heptane/MeCOOH 2049(vs) C6D6/MeCOOH 17.2 0.85 (t, 18H, CH3CH2, JHH= 7.3 Hz), 1.28 (q, 12H, CH3CH2, JHH= 7.3 Hz), 1.42 (m, 12H, CH2CH2P), 1.81 (m, 12H, CH2P), 2.27 (s, 6H, CH3COO) 14.1 (s, CH3CH2), 23.6 (s s, CH3COO), 24.1 (t, CH2P, JCP= 12.7 Hz), 25.0 (t, CH3CH2, JCP= 6.3 Hz), 25.6 (s, CH2CH2P), 179.3 (s, CH3COO), 198.7 (t, CO, JCP= 10.9 Hz t) 1988(vs) 1575(m)

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2.5. Catalytic activity of [Ru6(l-H)6(CO)10

(l-PHBu)-(l-PBu2)2(PBu3)2(l6-P)] (7)

The phosphido clusters (7)–(10) were formed from the catalytic precursors (1–4) when heated at high tem-perature under hydrogen. The catalytic activity of (7), the main cluster formed, was correlated with those of its precursors (1–4) to evaluate the role of phosphido clusters in the hydrogenations performed in the presence of the ruthenium complexes (1–4) as catalytic precursors.

Diﬀerent substrates were hydrogenated in the presence of (7) and the ruthenium precursors (1)–(4) (Table 2).

Complex (7) was catalytically active in the hydrogen-ation of the substrates tested even if its activity was lower than that of the other catalysts. Furthermore the cluster (7) was recovered unaltered at the end of these hydrogenations.

The low catalytic activity of (7) suggests the forma-tion of phosphido clusters as the cause of the gradual loss of activity of (1)–(4) in the course of the reduction of acetic acid at 180°C.

3. Conclusions

The phosphine substituted ruthenium carbonyl carb-oxylates (3) and (4) react, at low temperature (25–40°C) with a large excess of acetic acid to form new complexes containing monodentate acetato ligands. Further reac-tion of these intermediates leads to the formareac-tion of hydrogen and a mononuclear ruthenium (II) complex

(11) containing a chelato and a monodentate acetato lig-and. The same reactivity may be involved in the synthe-sis of (2) from (12) in the presence of acetic acid and a stoichiometric amount of PBu3[8].

The complex (11) is stable in the presence of acetic acid but easily reacts at room temperature with PPh3

giving a new complex containing two diﬀerent phosphi-nic ligands (13).

The reactivity of ruthenium complexes (3) and (4) changes when hydrogen and acetic acid are both pre-sent: (3) does not react up to 180°C while (4) gives (3) and (12) at 60 °C. The high hydrogen pressure (100 atm) hinders the formation of (11) from (3) or (4) in agreement with a process involving the formation of molecular hydrogen (Schemes 2 and 4).

Hydride ruthenium complexes (1), (5)–(10) and (14) are formed in the reactions of (2)–(4) with hydrogen and acetic acid but the presence of acetic acid reduces the rates of these reactions. These results conﬁrm the hypothesis that hydride ruthenium complexes are the ac-tive species in the catalytic hydrogenation of acetic acid

[1h]and suggest equilibria among H–Ru and CH3COO–

Ru species. In the presence of a large amount of acetic acid these equilibria are shifted towards the ruthenium carboxylates (2), (3) and (4).

The hydrogenation of acetic acid to ethyl acetate takes place at 140 °C in the presence of (3), at 160 °C with (4) and at 180 °C with (2). In these conditions (3) is always formed even if in diﬀerent amounts from (4) or (2) depending on the ruthenium precursor and reac-tion temperature employed. These results support the hypothesis that the same catalytically active intermedi-ate is formed from (2)–(4) in the course of acetic acid

Table 2

Hydrogenation of unsaturated organic substrates in the presence of ruthenium complexes

Catalyst Code Hydrogenation of (yield %)

Cyclohexenea _{Tiglic acid}b _Acetophenonec _{Acetic acid}d

Ru4H4(CO)8(PBu3)4 (1) 47.1 21.7 9.6 40.7

Ru(CO)2(MeCOO)2(PBu3)2 (2) 2.0 15.4 45.8 16.5

Ru2(CO)4(l-MeCOO)2(PBu3)2 (3) 22.2 98.2 65.7 34.4

Ru4(CO)8(l-MeCOO)4(PBu3)2 (4) 88.3 79.1 14.3 29.9

[Ru6(l-H)6(CO)10(l-PHBu)(l-PBu2)2(PBu3)2(l6-P)] (7) 0.4 0.3 1.0 2.9

Catalyst: 0.018 mmol Ru, substrate: 42.5 mmol, p(H2) = 130 atm, reaction time 22 h. a_{T = 60}

°C, hydrogenated product: cyclohexane.

b_{Solvent: 20 ml, toluene/ethanol (1:1), T = 100}

°C, hydrogenated product: 2-methylbutanoic acid.

c

T = 120°C, hydrogenated product: 1-phenylethanol.

d

Catalyst: 0.139 mmol Ru, substrate 433 mmol, T = 180°C, reaction time 48 h, hydrogenated product: ethyl acetate. Ru(CO)2(MeCOO)2(PBu3)2 H2 RuH(CO)2(MeCOO)(PBu3)2 MeCOOH

RuH(CO)2(MeCOO) (PBu3)2 H2 RuH2(CO)2(PBu3)2 MeCOOH

+ +

(2) (14)

(14) (5)

(8)

hydrogenation. Furthermore the hydride species are transformed into phosphido ruthenium cluster com-plexes such as (7) in the course of the reaction.

The polynuclear ruthenium complex (7) shows a cat-alytic activity lower than the precursors (1)–(4). As a consequence the loss of catalytic activity shown by the ruthenium complexes (1)–(4) in the reduction of acetic acid may be ascribed to this transformation[1b]. Other authors attribute the deactivation of catalytic systems to the partial loss of the alkyl groups of the phosphine lig-ands with formation of phosphido clusters[14].

4. Experimental 4.1. Instruments

Gas chromatographyc analyses (glc) were performed with a Perkin–Elmer Sigma 1 system coupled with a Per-kin–Elmer Sigma 10 computer or using a Shimadzu GC-14A chromatographic system coupled with a Shimadzu C-R4A computer. Both systems were equipped with a FID detector.

The following packed columns (2 m) were used: PPG (‘‘Polypropylenglicol’’ LB-550-X on Chromosorb W at 15%), FFAP (‘‘free fatty acids phase’’ on Chromosorb G AW-DMCS at 5%), CW (‘‘Carbowax 20M’’ on Chro-mosorb W at 15%).

The conversion of acetic acid to ethyl acetate was evaluated using a calibration curve.

Glc-ms spectra were collected using a Shimadzu GC-MS QP2000 instrument or a Carlo Erba QMD 1000 GC-MS system equipped with capillary columns: a SPB-1TM (Supelco column, 30 m, internal diameter 0.25 mm) or a ATTM-1 (Alltech column, 30 m, internal diameter 0.25 mm).

A Perkin–Elmer SCIEX API 365, a turbo ion spray system, was employed to obtain the MS spectra of the ruthenium complex.

Infrared spectra were recorded, at room temperature and pressure, with a Perkin–Elmer model 1760-X FTIR spectrophotometer. Liquid products and solutions were analysed using KBr or CaF2cells having 0.1 mm path.

Solid samples were mulled with KBr. The IR spectra collected at high pressure and temperature were re-corded with a Perkin–Elmer spectrophotometer mod. 580B Data System using the cell previously described

[1d].

Multinuclear NMR spectra were registered using a Varian VXR300 spectrometer operating at 299.944 MHz for 1H, at 75.429 MHz for 13C and at 121.421 MHz for31P NMR; tetramethylsilane was used as exter-nal standard for1H- and13C NMR spectra. In the31P NMR spectra, downﬁeld values from external H3PO4

(85%) were taken as positive.13C- and31P NMR spectra were recorded as proton decoupled spectra.

Elemental analyses were performed with a Perkin–El-mer model 240 C system.

Tlc separations were performed on silica gel (thick-ness 2 mm) or alumina (thick(thick-ness 1.5 mm) chromato-graphic plate (Merck) with ﬂuorescent indicator F254.

4.2. Test procedure

4.2.1. Reactivity of ruthenium complexes by IR spectro-scopy

All tests were performed in a stainless steel autoclave (125 ml) equipped with two stopcocks and a high-pressure gauge. Air was evacuated from the vessel, the solution of reactants was introduced by suction then the gas (nitrogen or hydrogen) was transferred from a cylinder up to the pressure required.

The vessel was connected through a stainless-steel low volume (2 ml) coil to the IR cell, equipped with NaCl windows, capable of withstanding high pressure (200 atm), which could be heated up to 200°C. The coil was kept at the same temperature of the system. The solution present in the autoclave was transferred into the IR cell under reaction conditions and examined by IR spectroscopy. All spectra were recorded after abun-dant ﬂushing of both coil and IR cell with the solution present in the vessel. The solvent (n-heptane) bands were compensated using a variable-path IR cell.

4.2.2. Catalytic hydrogenation experiments

A solution containing the catalytic precursor, solvent, substrate and hydrogen was introduced into an evacu-ated stainless steel autoclave (150 ml) containing a steel cylinder (72 ml) to reduce the free volume. The tests of acetic acid hydrogenation were carried out in a ‘‘Hastel-loy C’’ stainless steel rocking autoclave (125 ml). The vessel was placed in a thermostatic oil bath set at the de-sired temperature (±1 °C) and rocked for the preﬁxed time. The amounts of catalytic precursor, solvent, sub-strate and hydrogen are reported in Table 2.

At the end of the reaction, the reactor was cooled, the gases vented and the solution analysed by glc. The iden-tity of the products was conﬁrmed by glc-ms analysis. 4.3. Materials

All preparations and manipulations were routinely performed under dry nitrogen using Schlenk tube techniques.

Reagents and solvents were puriﬁed and dried as re-ported. Acetic acid was distilled under nitrogen (bp 118 °C). Cyclohexene was eluted through activated Al2O3 (70–230 mesh) and rectiﬁed under nitrogen (bp

83 °C). Acetophenone was distilled prior to use (bp 82 °C/15 mmHg). Toluene was reﬂuxed on sodium metal, then reﬂuxed and distilled on LiAlH4. n-Heptane was

(9)

a solution of KMnO4in H2SO4(10%), dried on

anhy-drous CaCl2, reﬂuxed on sodium and distilled on

LiAlH4. Methanol, dried as reported by Vogel [15],

had bp 65°C. Tri-n-butylphosphine was distilled under nitrogen prior to use (bp 158–160°C/60 mmHg).

All other solvents and chemicals were reagent grade and used without further puriﬁcation.

4.3.1. Synthesis of ruthenium complexes

Complexes Ru4H4(CO)8(PBu3)4 (1) [16], Ru(CO)2

-(MeCOO)2(PBu3)2 (2) [8], Ru2(CO)4(l-MeCOO)2

(P-Bu3)2 (3) [17], Ru4(CO)8(l-MeCOO)4(PBu3)2 (4) [12],

RuH2(CO)2(PBu3)2 (5) [1d], Ru4H4(CO)9(PBu3)3 (6)

[16], [Ru6(l-H)6(CO)10(l-PHBu)(l-PBu2)2(PBu3)2(l6-P)]

(7) and [Ru2(CO)4(l-MeCOO)2]n(12)[17]were prepared

as described in the literature. The 1H-, 31P- and 13C NMR spectra of (1)–(4) and (6) were performed using the same solvents (C6D6) employed in experimental tests

with the aim to facilitate their identiﬁcation in the course of the reactions (Table 3).

4.4. Reactivity of ruthenium complexes in the presence of acetic acid: IR study

A solution of the system under examination was introduced in the autoclave, then N2(5 atm) was added.

IR spectra were recorded under reaction conditions after heating the solution at the pre-ﬁxed temperature for the selected time.

4.4.1. Ru2(CO)4(l-MeCOO)2(PBu3)2(3) with MeCOOH

A solution of (3) (120 mg, 0.144 mmol) and acetic acid (0.5 ml, 8.734 mmol) in n-heptane (60 ml) was heated under nitrogen (5 atm at 20°C) in the tempera-ture range 40–150°C.

The reaction was followed as reported in the general procedure.

A solution of (4) (119 mg, 93.8 lmol) and acetic acid (0.5 ml, 8.734 mmol) in n-heptane (60 ml) was heated under nitrogen (5 atm at 20°C) in the temperature range 20–140°C.

The solution recovered at the end of the experiment was evaporated to dryness, the residue dissolved in C6D6 and analysed by IR,1H- and 31P NMR. Several

singlets were present in the31P NMR spectrum besides the signals at 43.5 ppm due to (11), at 17.4 (2), 38.5, 43.1, 44.6, 45.4, 53.3 ppm.

4.4.3. Ru(CO)2(MeCOO)2(PBu3)2(2) with MeCOOH

A solution of (2) (190 mg, 0.279 mmol) and acetic acid (0.5 ml, 8.734 mmol) in n-heptane (60 ml) was

heated under nitrogen (5 atm at 20°C) in the tempera-ture range 20–150°C.

A sample of the solution was collected after heating at 40 °C. After evaporation of the solvent and acetic acid under reduced pressure the residue was dissolved in n-heptane and analysed by IR spectroscopy: only the bands at 2041(vs), 1971(vs) and 1628(m) cm1 attrib-utable to the starting complex were present. The 1H-,

31

P- and13C NMR spectra were recorded on the residue dissolved in C6D6or in C6D6containing the same

con-centration of MeCOOH used for the reactivity tests. The absorptions are reported in Table 1.

4.5. Reactivity of ruthenium complexes in the presence of acetic acid: NMR study

A solution of (3) (24 mg, 0.029 mmol) and acetic acid (0.1 ml, 1.747 mmol) in C6D6(1 ml) was introduced,

un-der nitrogen, into a NMR sample tube.

After 56 h at 40 °C two singlet at 43.5 (70.4%) and 17.2 ppm (29.6%) were present in the 31P NMR spec-trum while broad bands at 2060(vs), 1990(vs), 1950(vw) and 1575(w) cm1were present in the IR spectrum.

The solution was evaporated to dryness and the resi-due dissolved in C6D6: a broad singlet was present in the 31

P NMR spectrum at 44.4 ppm (70.4%), attributed to (11), together with a singlet at 17.4 ppm (29.6%), attrib-uted to (2).

In the 1H NMR spectrum (C6D6) signals attributed

to (11) were present at d 0.77 (m, 9H, CH3, PBu3),

1.16 (m, 6H, CH3CH2, PBu3), 1.30 (m, 6H, EtCH2,

PBu3), 1.63 (m, 6H, PCH2, PBu3), 2.06 (s broad, 6H,

CH3COO) ppm and other resonances attributed to (2)

at d 0.90 (t, 18H, CH3, PBu3, JHH= 7.1 Hz), 1.30 (m,

12H, CH3CH2, PBu3), 1.53 (m, 12H, EtCH2, PBu3),

1.87 (m, 12H, PCH2, PBu3), 2.27 (s, 6H, CH3COO)

ppm.

The 13C NMR spectrum (C6D6) showed signals

attributed to (11) at d 13.4 (s, CH3, PBu3), 20.1 (s,

l-CH3COO), 23.5 (s, CH3COO), 24.1 (m, PCH2, PBu3),

24.3 (s, CH3CH2, PBu3), 24.9 (m, EtCH2, PBu3), 174.5

(s, COO), 183.0 (s, l-COO), 195.9 (m, CO) ppm and other resonances attributed to (2) at d 13.8 (s, CH3,

PBu3), 23.7 (s, CH3COO), 23.9 (t, PCH2, PBu3,

JPC= 12.7 Hz), 24.8 (t, CH3CH2, PBu3, JPC= 6.3 Hz),

25.4 (s, EtCH2, PBu3), 175.9 (s, COO), 199.0 (t, CO,

JPC= 10.9 Hz) ppm.

The IR spectrum of the same sample dissolved in n-pentane showed the bands of the complex (11) at 2057(vs), 1987(vs), 1603(w), 1563(vw) cm1 and those of the complex (2) at 2041(s), 1971(s), 1628(vw) cm1.

The sample kept for 24 h at room temperature in a C6D6solution, without acetic acid, was transformed in

(10)

several compounds as shown by various singlets present in the31P NMR spectrum as above reported.

A solution of (4) (18.4 mg, 0.014 mmol) and acetic acid (0.1 ml, 1.747 mmol) in C6D6 (1 ml) was

intro-duced, under nitrogen, into a NMR sample tube. After 1 h at room temperature new singlets were present in the

31

P NMR spectrum at 43.5 (11), 38.8 and 13.4 ppm.

The conversion of (4) was 24.5% after 1 h and 100% after 15 days at room temperature. The singlet of (11) at 43.5 ppm was the main resonance (87.2%) and a yellow residue was present in the sample tube. The solid was separated and the IR spectrum recorded as nujol mull showed bands at 2055(s), 1995(vs), 1970(vs), 1910(s) and 1555(vs) cm1 attributable to (12).

The solution was evaporated to dryness and the res-idue dissolved in C6D6and spectroscopically

character-Table 3

NMR data of ruthenium complexes in C6D6as solvent

Complex 31_{P NMR} 1_{H NMR} 13_{C NMR} (1)a _16.6 16.60 (qt,4H, HRu, JHP= 6.0 Hz) 14.0 (s, CH3) 0.91 (t, 36H, CH3, JHH= 7.3 Hz) 24.9 (t, CH3CH2, JCP= 6.0 Hz) 1.34 (q, 24H, CH3CH2, JHH= 7.3 Hz) 26.6 (s, CH2CH2P) 1.52 (m, 24H, CH2CH2P) 31.7 (t, CH2P, JCP= 10.4 Hz) 1.86 (m, 24H, CH2P) 202.2 (m, CO) (3)b 8.3 0.89 (t, 18H, CH3CH2, JHH= 6,8 Hz) 13.8 (s, CH3CH2) 1.35 (q, 12H, CH3CH2, JHH= 6.8 Hz) 23.4 (s, CH3COO) 1.56 (m, 12H, CH2CH2P) 24.6 (t, CH2P, JCP= 7.9 Hz) 1.81 (m, 12H, CH2P) 24.8 (t, CH3CH2, JCP= 5.6 Hz) 1.85 (s, 6H, CH3COO) 25.6 (s, CH2CH2P) 186.3 (t, COO, JCP= 7.9 Hz) 208.3 (m, CO) (4)c _6.1 _{0.85 (t, 18H, CH} 3CH2, JHH= 6,8 Hz) 13.8 (s, CH3CH2) 1.28 (q, 12H, CH3CH2, JHH= 6,8 Hz) 23.5 (s, CH3COO) 1.43 (m, 12H, CH2CH2P) 23.8 (d, CH2P, JCP= 12.7 Hz) 1.67 (m 12H, CH2P) 24.7 (d, CH3CH2, JCP= 6.5 Hz) 2.12 (s, 12H, CH3COO) 25.4 (s, CH2CH2P) 185.3 (d, COO, JCP= 6.5 Hz) 203.5 (d, CO, JC–P= 10.9 Hz) 205.4 (d, CO, JC–P= 10.9 Hz) (6)a 20.7 16.85 (m, 4H, HRu) n.d. 22.7 16.69 (m, 4H, HRu) 0.97 (t, 27H, CH3, JHH= 7,3 Hz) 1.43 (m, 18H, CH3CH2) 1.60 (m, 18H, CH2CH2P) 1.89 (m, 18H, CH2P) (11) 44.4d _{0.77 (m, 9H, CH} 3, PBu3), 1.16 (m, 6H, CH3CH2, PBu3), 1.30 (m, 6H, EtCH2, PBu3), 1.63 (m, 6H, PCH2,

PBu3), 2.06 (s broad, 6H, CH3COO)

13.4 (s, CH3, PBu3), 20.1 (s,

l-C3COO), 23.5 (s, C3COO), 24.1 (m,

PC2PBu3), 24.3 (s, CH3C2PBu3), 24.9

(m, EtC2PBu3), 174.5 (s, COO), 183.0

(s, l-COO), 195.9 (m, CO) (13) 23.0 (AB, 1P, JPP= 317.4 Hz, PnBu3), 29.7 (AB, 1P, JPP= 317.4 Hz, PPh3) 0.82 (t, 9H, CH3CH2, JHH= 7.3 Hz), 1.24 (q, 6H, CH3CH2, JHH= 7.3 Hz), 1.49 (m, 6H, CH2CH2P), 1.88 (s, 6H, CH3COO), 1.94 (m, 6H, CH2P), 7.02 (m, 3H, Hp, PPh3), 7.10 (m, 6H, Hm, PPh3), 7.96 (m, 6H, Ho, PPh3) 13.5 (s, CH3CH2), 23.2 (s, CH3COO), 24.0 (d, CH2P, JCP= 22.6 Hz), 24.5 (d, CH3CH2, JCP= 11.3 Hz), 25.1 (s, CH2CH2P), 130.3 (s, Cm, PPh3), 131.4 (s, Cp, PPh3), 132.3 (d, Ci, PPh3, JCP= 11.3 Hz), 134.6 (d, Co, PPh3, JCP= 9.0 Hz), 175.8 (s, CH3COO), 198.8 (pt, CO, JCP= 11.3 Hz)

a _{NMR data are partially reported}_[16]_. b_{NMR data are reported in CDCl}

3as solvent[17,18]. c_{NMR data are partially reported}_[12]_.

d

(11)

ised as complex (11). NMR data are reported in

Table 2.

In the IR spectrum (n-pentane) were present bands at 2057(vs), 1987(vs), 1563(vw, broad) cm1 due to (11).

4.6. Synthesis of Ru(CO)2(MeCOO)2(PBu3)(PPh3) (13)

Triphenylphosphine (15.21 mg, 0.058 mmol) was added to the solution containing (11) in C6D6 (PPh3/

Ru = 1/1, molar ratio) at room temperature and the mixture monitored by31P NMR.

After 3 h a second-order AB spin system with reso-nances at d 23.9 (d, 1P, PBu3, JPP= 311.6 Hz) and

30.5 (d, 1P, PPh3, JPP= 311.6 Hz) ppm was present in

the31P NMR spectrum while the intensity of the singlet attributable to (11) was very low. The conversion was 90% after 5 h.

The solvent was evaporated and the new complex, recrystallized from n-pentane at 0 °C as white crystals and spectroscopically characterised.

The IR spectrum of (13) (n-pentane), in the 2200– 1500 cm1 region, showed bands at 2044(vs), 1982(vs) and 1626(m) cm1.

In the1H NMR spectrum of (13) (C6D6) signals were

present at d: 0.79 (t, 9H, CH3, PBu3, JHH= 7.3 Hz), 1.21

(q, 6H, CH3CH2, PBu3, JHH= 7.3 Hz), 1.46 (m, 6H,

EtCH2, PBu3), 1.84 (s, 6H, CH3COO), 1.91 (m, 6H,

PCH2, PBu3), 7.02 (m, 6H, PPh3), 7.70 (m, 3H, PPh3),

7.93 (m, 6H, PPh3) ppm.

In the13C NMR spectrum of (13) (C6D6) signals were

present at d: 13.5 (s, CH3, PBu3), 23.2 (s, CH3COO),

24.0 (d, PCH2, PBu3, JCP= 22.6 Hz), 24.5 (d, CH3CH2,

PBu3, JCP= 11.3 Hz), 25.1 (s, EtCH2, PBu3), 130.3 (s,

Cm, PPh3), 131.4 (s, Cp, PPh3), 132.3 (d, Cipso, PPh3, JCP= 11.3 Hz), 134.6 (d, Co, PPh3, JCP= 9.0 Hz), 175.8 (s, COO), 198.5 (m, CO) ppm. MS spectrum of (13) m/z (%): 740 (10) [M]+, 712 (2) [M–CO]+, 681 (100) [M–CH3COO] + , 653 (20) [M– CH3COO–CO] +

, 622 (5) [Ru(PPh3)(PBu3)(CO)2] +

(cen-tres of each ruthenium cluster peaks are reported)

Elemental analysis of (13), for C36H48O6P2Ru: % C

58.3 (58.45), % H 6.6 (6.54).

4.7. Reactivity of ruthenium complexes with acetic acid and hydrogen

A n-heptane solution of each ruthenium complex and acetic acid was introduced in the autoclave under dry nitrogen, then H2(100 atm) was added.

IR spectra were recorded in the reaction conditions after heating the solution at the pre-ﬁxed temperature for the selected time.

Samples of the solution examined by IR spectros-copy, were collected and analysed by glc using a FFAP column kept at 50°C for 7 min, then heated up to 130

°C at a rate of 30 °C/min and kept at this temperature for 10 min. The peaks due to acetic acid and ethyl ace-tate were identiﬁed and quantiﬁed.

4.7.1. Ru2(CO)4(l-MeCOO)2(PBu3)2(3)

A solution of (3) (120 mg, 0.144 mmol) and acetic acid (0.5 ml, 8.734 mmol) in n-heptane (60 ml) under hydrogen (100 atm at 20°C) was heated in the temper-ature range 20–180°C.

At the end of the reaction the vessel was cooled to room temperature, the solvent evaporated under re-duced pressure and the solid recovered and separated by preparative tlc on alumina using n-hexane as eluant. Traces of hydride clusters (7), (9) and (10) were obtained.

4.7.2. Ru4(CO)8(l-MeCOO)4(PBu3)2(4)

A solution of (4) (119 mg, 93.8 lmol) and acetic acid (0.5 ml, 8.734 mmol) in n-heptane (60 ml) under hydro-gen (100 atm at 20 °C) was heated in the temperature range 40–180 °C.

4.7.3. Ru(CO)2(MeCOO)2(PBu3)2(2)

A solution of (2) (190 mg, 0.279 mmol) and acetic acid (0.5 ml, 8.734 mmol) in n-heptane (60 ml) under hydrogen (100 atm at 20°C) was heated in the temper-ature range 20–180°C.

4.8. Catalytic hydrogenation experiments: analyses of the hydrogenation products

The experimental conditions and the results are re-ported in Table 2.

The products present at the end of the hydrogenation were analysed by glc and their identities conﬁrmed by glc-ms.

The solvent, the starting substrate and the reaction products were separated and quantiﬁed using the fol-lowing conditions:

Cyclohexene: a PPG column was kept at 40 °C for 25 min.

Tiglic acid: a FFAP column was kept at 140 °C per 15 min.

Acetophenone: a CW column was kept at 60 °C for 5 min, then heated up to 160°C at a rate of 20 °C/min and kept at this temperature for 10 min.

Acetic acid: a FFAP column was kept at 50 °C for 7 min, heated up to 130°C at a rate of 30 °C/min and kept at this temperature for 10 min.

(12)

In the hydrogenation of acetic acid the yields were eval-uated using calibration curves obtained from mixtures of ethyl acetate, ethanol and acetic acid of known com-position. In all other tests conversions were evaluated without response factors corrections.

Acknowledgements

The authors thank the University of Florence and the Ministero della Istruzione, Universita` e Ricerca (M.I.U.R.), Programmi di Ricerca Scientifica di Note-vole Interesse Nazionale, Cofinanziamento 2003–04, for financial support. We also thank Prof. Mario Bian-chi for stimulating suggestions and discussions.

References

[1] (a) P. Frediani, U. Matteoli, G. Menchi, M. Bianchi, F. Piacenti, Gazz. Chim. Ital. 115 (1985) 365;

(b) U. Matteoli, G. Menchi, M. Bianchi, P. Frediani, F. Piacenti, Gazz. Chim. Ital. 115 (1985) 603;

(c) P. Frediani, M. Bianchi, F. Piacenti, S. Ianelli, M. Nardelli, Inorg. Chem. 26 (1987) 1592;

(d) P. Frediani, M. Bianchi, A. Salvini, F. Piacenti, S. Ianelli, M. Nardelli, J. Chem. Soc., Dalton Trans. (1990) 165;

(e) P. Frediani, M. Bianchi, A. Salvini, F. Piacenti, S. Ianelli, M. Nardelli, J. Chem. Soc., Dalton Trans. (1990) 1705;

(f) P. Frediani, M. Bianchi, A. Salvini, F. Piacenti, J. Chem. Soc., Dalton Trans. (1990) 3663;

(g) P. Frediani, C. Faggi, S. Papaleo, A. Salvini, M. Bianchi, F. Piacenti, S. Ianelli, M. Nardelli, J. Organomet. Chem. 536 (1997) 123;

(h) P. Frediani, C. Faggi, A. Salvini, M. Bianchi, F. Piacenti, Inorg. Chim. Acta 272 (1998) 141.

[2] (a) M. Bianchi, G. Menchi, F. Francalanci, F. Piacenti, U. Matteoli, P. Frediani, C. Botteghi, J. Organomet. Chem. 188 (1980) 109;

(b) M. Bianchi, F. Piacenti, P. Frediani, U. Matteoli, C. Botteghi, S. Gladiali, E. Benedetti, J. Organomet. Chem. 141 (1977) 107;

(c) M. Bianchi, F. Piacenti, G. Menchi, P. Frediani, U. Matteoli, C. Botteghi, S. Gladiali, E. Benedetti, Chim. Ind. (Milan) 60 (1978) 588.

[3] U. Matteoli, M. Bianchi, G. Menchi, P. Frediani, F. Piacenti, J. Mol. Catal. 22 (1984) 353.

[4] T. Ohta, H. Takaya, R. Noyori, Inorg. Chem. 27 (1988) 566. [5] (a) A. Spencer, G. Wilkinson, J. Chem. Soc., Dalton Trans.

(1974) 786;

(b) A. Dobson, S.D. Robinson, Inorg. Chem. 16 (1977) 1321; (c) S.D. Robinson, M.F. Uttley, J. Chem. Soc., Dalton Trans. (1973) 1912;

(d) A. Dobson, S.D. Robinson, M.F. Uttley, J. Chem. Soc., Dalton Trans. (1975) 370.

[6] C.J. Creswell, A. Dobson, D.S. Moore, S.D. Robinson, Inorg. Chem. 18 (1979) 2055.

[7] M. Rotem, I. Goldberg, U. Shmueli, Y. Shvo, J. Organomet. Chem. 314 (1986) 185.

[8] M. Bianchi, P. Frediani, U. Matteoli, G. Menchi, F. Piacenti, G. Petrucci, J. Organomet. Chem. 259 (1983) 207.

[9] G. Fachinetti, T. Funaioli, L. Lecci, F. Marchetti, Inorg. Chem. 35 (1996) 7217.

[10] T. Funaioli, F. Marchetti, G. Fachinetti, Chem. Commun. (1999) 2043.

[11] D.W. Krassowski, J.H. Nelson, K.R. Brower, D. Hauenstein, R.A. Jacobson, Inorg. Chem. 27 (1988) 4294.

[12] M. Bianchi, U. Matteoli, P. Frediani, F. Piacenti, M. Nardelli, G. Pelizzi, Chim. Ind. (Milan) 63 (1981) 475.

[13] P. Frediani, A. Salvini, M. Bianchi, F. Piacenti, J. Organomet. Chem. 454 (1993) C17.

[14] P.E. Garrou, Chem. Rev. 85 (1985) 60.

[15] A. Vogel, VogelÕs Textbook of Practical Organic Chemistry, fourth ed., Longmans, London, 1978.

[16] F. Piacenti, M. Bianchi, P. Frediani, E. Benedetti, Inorg. Chem. 10 (1971) 2759.

[17] G.R. Crooks, B.F.G. Johnson, J. Lewis, I.G. Williams, G. Gamlen, J. Chem. Soc. A (1969) 2761.

[18] U. Matteoli, G. Menchi, M. Bianchi, F. Piacenti, J. Mol. Catal. 64 (1991) 257.